1. Field of the Invention
The present invention relates to an auto focus camera, and, more particularly, to an auto focus camera which is adapted for use in an auto focus (AF) apparatus of a camera that drives a photographic lens to a focal position based on the output of focal point detecting means.
2. Description of the Related Art
While various functions of a camera become automated, the camera technology for focusing on a subject has been developed toward dealing with a subject that is not easy to cope with. A method of preventing the focal point from shifting due to the movement of a subject moving in the direction of the optical axis of a camera during a release time lag at the time of photographing such a moving subject, is disclosed in, for example, Unexamined Japanese Patent Publication No. 63-159817. The apparatus disclosed in this Japanese publication is structured as follows.
A light-emitting element driver causes an IRED (infrared light-emitting diode) to emit light to a subject via a projecting lens. The light reflected from the subject is collected by a light-receiving lens and is guided to a PSD (position sensitive detector), which in turn outputs a signal current according to the incident position of the reflected light. This signal current is then supplied to a distance computing circuit to compute the distance to the subject. The distance measuring operation is performed plural times in response to a first release signal in accordance with the action of a timing circuit, and the individual measuring results are memorized. The speed of the moving subject is detected by computing how much the subject has moved within a predetermined time. The position of the subject at the beginning of exposure is predicted based on this speed, and the photographic lens is driven according to the prediction.
In other fields than the field of cameras, Unexamined Japanese Patent Publication No. 62-232571 discloses a moving-object distance measuring technique which emits infrared rays to a subject at different timings, determines the speed of the subject according to the measuring results based on the reflection signals, and prevents the focal point from shifting due to the movement of the subject during a release time lag.
However, the acquiring of the speed by operating a distance-measuring apparatus for an ordinary camera twice at different timings is not satisfactory unless a measuring error hardly occurs or the interval between the two measuring timings is large. This method is therefore not so practical.
Because the actual distance measuring inevitably involves an error and in view of the shutter lag, taking a long time to acquire the speed of a subject is not feasible even in the light of the purpose of measuring the distance to a moving subject.
Further, in a so-called active type AF (auto focus) device, to emit light and measure the subject distance according to the reflected light, generally, some attempts have been made, such as increasing the number of light emissions to improve the distance-measuring accuracy and averaging the results. While increasing the number of light emissions improves the distance-measuring accuracy, the measuring time or the time lag becomes longer.